A measure of the performance of a lamp can be given by the efficacy of the lamp in lumens/Watt, i.e. the luminous flux produced by the lamp as a ratio of the power required to produce that luminous flux. For many lighting applications, a constant light-flux—and therefore a constant efficacy—is desirable. For lamps such as high-intensity gas-discharge (HID) lamps that operate by applying an alternating voltage across two electrodes, some fluctuation can occur at the relatively high operating frequency of the lamp. As the lamp ages, the electrode topology changes, causing variations in the length of the discharge arc and an associated fluctuation in lamp voltage, since the lamp voltage is directly related to the length of the discharge arc. It follows that the light output also fluctuates, since the light output is closely related to the lamp voltage. While any fluctuations in light output at these high frequencies cannot be perceived by a human observer, they indicate a drop in performance of the lamp and are therefore undesirable for that reason. Various lamp driver realisations address this problem, for example by briefly increasing the lamp power prior to a commutation of the lamp voltage.
However, fluctuation in the light output at lower frequencies in the range of several tens of Hz can indeed be perceived as annoying. In particular, light output fluctuations in the range 5-20 Hz can be easily detected by a human observer since the eye is particularly sensitive to light fluctuations in this frequency range. An increase or decrease in the light output of only 0.5% can be noticeable. Such perceptible fluctuations can be physically or mechanically induced by an arc movement or displacement inside the discharge vessel, and can result in noticeable beam pattern instabilities. Automotive HID headlamp systems can exhibit such beam pattern instabilities during driving over a bumpy road, e.g. railroad crossing or cobblestone pavement, or in situations when the lamp is subjected to mechanical impact, e.g. when the motor is re-started, when a car door is slammed shut, etc.
In present-day automotive front-lighting systems, the lamp driver operates the lamp in steady state in such a way that the lamp power is kept essentially constant. Different algorithms can be used to stabilize the lamp power, depending on the driver hardware. If the lamp is subject to mechanical impact, for example in one of the situations mentioned above, the arc inside the discharge vessel is displaced, leading to lamp voltage modulations. If the lamp is suddenly displaced, the discharge arc—a plasma extending between the two electrode tips—is moved relative to the discharge vessel. Because of its high temperature, the discharge arc has a lower density than the surrounding gas in the discharge vessel and is therefore lighter. If the lamp is subject to an abrupt downward displacement, for example, the discharge arc is also deflected downwards by the cooler (and therefore heavier) surrounding gas and is therefore shortened. For the same reason, an abrupt upward displacement of the lamp causes the discharge arc to be pushed upward by the cooler, heavier surrounding gas, and is therefore lengthened. As a result, the discharge arc can be ‘stretched’ or ‘compressed’, depending on the direction of the spatial displacement of the lamp. The lamp voltage increases or decreases accordingly. During this time, the light output fluctuates to follow the fluctuations in lamp voltage. When a ‘slow’ power control algorithm is used by the lamp driver, the lamp voltage modulations will lead to lamp power modulations, and these result in a modulation of the integral light flux of the lamp. This modulated light flux leads to a perceptible forefront flicker (FFF) in the beam close to the front of the vehicle. A fast power control algorithm, which is also sometimes implemented in drivers nowadays, is associated with a better performance and results in less severe light flux modulations. Even for such a fast power control algorithm, visible forefront flicker can remain a problem owing to the fluctuation in lamp efficacy while the power control algorithm adjusts the lamp power.
Therefore, it is an object of the invention to provide an improved way of driving an arc-discharge lamp to reduce perceptible forefront beam pattern instabilities.